Enhancement of methanogenic activity in volumetrically undersized reactor by mesophilic co-digestion of sewage sludge and aqueous residue

To date, energy recovery from biological sewage sludge (BSS) with anaerobic digestion is very popular. However, it can often happen that anaerobic reactors are volumetrically undersized thus reducing performance in terms of biogas production. A continuous flow pilot-scale plant was used to investigate, for the first time, the effects of mesophilic anaerobic co-digestion (MA-CoD) of sewage sludge and aqueous residue (AR) from a biosolids treatment plant (BTP) on methanogenic activity in low hydraulic retention time (HRT) conditions (to simulate the under-sizing of the reactor). The results showed that the digestate is always more rapidly biodegrada-ble than the matrices fed while particulate COD hydrolyzed (12 ± 1.3 %) is independent of the quantity of AR dosed. Feeding over 35% of soluble OLR, the total VFAs in the system strongly decreased, despite the low HRT. In correspondence of higher dosages of AR, the percentage of CH4 increased up to 77-78% and CO2 CH4-1 ratio decreased to 0.25 ± 0.2. Specific methane produc-tion increased from 0.09 ± 0.01 m3CH4 kgCODremoved-1 with BSS alone to 0.28 ± 0.01 m3CH4CH4 kgCODremoved-1 in case of BSS co-digested with AR. Moreover, co-digestion with AR from BTP al-lowed to enhance continuous specific methanogenic activity from 1.76 ± 0.02 m3CH4 tVSS-1 d-1 to 6.48 ± 0.88 m3CH4 tVSS-1 d-1. Therefore, the MACoD of BSS and AR from BTS could be a good solu-tion to enhance methanogenic activity in volumetrically undersized anaerobic digester with re-duced HRT.